var VSHADER_SOURCE =
  'attribute vec4 a_Position;\n' +
  'attribute vec2 a_TexCoord;\n' +
  'varying vec2 v_TexCoord;\n' +
  'void main() {\n' +
  '  gl_Position = a_Position;\n' +
  '  v_TexCoord = a_TexCoord;\n' +
  '}\n';

// Fragment shader program
var FSHADER_SOURCE =
  '#ifdef GL_ES\n' +
  'precision mediump float;\n' +
  '#endif\n' +
  'uniform sampler2D u_Sampler;\n' +
  'varying vec2 v_TexCoord;\n' +
  'void main() {\n' +
  '  gl_FragColor = texture2D(u_Sampler, v_TexCoord);\n' +
  '}\n';

function main() {
    const canvas = document.getElementById('webgl')
    const gl = getWebGLContext(canvas)
    if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
        console.log('init shader error');
    }

    let n = initVertexBuffers(gl)

    gl.clearColor(0, 0, 0, 1);

    //新： 配置纹理
    if (!initTextures(gl, n)) {
        console.log('init texture error');
    }

}
function initVertexBuffers(gl) {
    var verticesTexCoords = new Float32Array([
      // Vertex coordinates, texture coordinate
      -0.5,  0.5,   0.0, 1.0,
      -0.5, -0.5,   0.0, 0.0,
       0.5,  0.5,   1.0, 1.0,
       0.5, -0.5,   1.0, 0.0,
    ]);
    var n = 4; // The number of vertices
  
    // Create the buffer object
    var vertexTexCoordBuffer = gl.createBuffer();
    if (!vertexTexCoordBuffer) {
      console.log('Failed to create the buffer object');
      return -1;
    }
  
    // Bind the buffer object to target
    gl.bindBuffer(gl.ARRAY_BUFFER, vertexTexCoordBuffer);
    gl.bufferData(gl.ARRAY_BUFFER, verticesTexCoords, gl.STATIC_DRAW);
  
    var FSIZE = verticesTexCoords.BYTES_PER_ELEMENT;
    //Get the storage location of a_Position, assign and enable buffer
    var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
    if (a_Position < 0) {
      console.log('Failed to get the storage location of a_Position');
      return -1;
    }
    gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, FSIZE * 4, 0);
    gl.enableVertexAttribArray(a_Position);  // Enable the assignment of the buffer object
  
    // Get the storage location of a_TexCoord
    var a_TexCoord = gl.getAttribLocation(gl.program, 'a_TexCoord');
    if (a_TexCoord < 0) {
      console.log('Failed to get the storage location of a_TexCoord');
      return -1;
    }
    // Assign the buffer object to a_TexCoord variable
    gl.vertexAttribPointer(a_TexCoord, 2, gl.FLOAT, false, FSIZE * 4, FSIZE * 2);
    gl.enableVertexAttribArray(a_TexCoord);  // Enable the assignment of the buffer object
  
    return n;
  }


function initTextures(gl, n) {
    var texture = gl.createTexture();   // Create a texture object
    if (!texture ) {
        console.log('Failed to create the texture object');
        return false;
    }

    // Get the storage location of u_Sampler
    var u_Sampler = gl.getUniformLocation(gl.program, 'u_Sampler');
    if (!u_Sampler) {
        console.log('Failed to get the storage location of u_Sampler');
        return false;
    }
    var image = new Image();  // Create the image object

    if (!image) {
        console.log('Failed to create the image object');
        return false;
    }
    // Register the event handler to be called on loading an image
    image.onload = function () { loadTexture(gl, n, texture, u_Sampler, image); };
    // Tell the browser to load an image
    image.src = 'sky.jpg';

    return true;
}

function loadTexture(gl, n, texture, u_Sampler, image) {
    gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, 1); // Flip the image's y axis
    // Enable texture unit0
    gl.activeTexture(gl.TEXTURE0);
    // Bind the texture object to the target
    gl.bindTexture(gl.TEXTURE_2D, texture);

    // Set the texture parameters
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
    // Set the texture image
    gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGB, gl.RGB, gl.UNSIGNED_BYTE, image);

    // Set the texture unit 0 to the sampler
    gl.uniform1i(u_Sampler, 0);

    gl.clear(gl.COLOR_BUFFER_BIT);   // Clear <canvas>

    gl.drawArrays(gl.TRIANGLE_STRIP, 0, n); // Draw the rectangle
}
